1. Field of the Invention
The present invention relates to a method of preventing skid of wheels of a vehicle, in which the slip rate of a wheel as well as the acceleration and deceleration thereof is added as one of essential factors for controlling the brake torque.
2. Brief Description of the Prior Art
When a vehicle is stopped abruptly, the wheels of the vehicle may be locked if the braking input to these wheels is too large. Once the wheels are locked during the braking, not only the braking efficiency is lowered but also the directional stability and steerability of the vehicle are lost dangerously.
The above stated danger can be avoided by automatically controlling the braking torque exerted on the wheel, irrespective of the braking input provided by the driver, such that the slip rate of the wheel falls within an appropriate range, e.g. between 15 and 20%.
To this end, various anti-skid braking method have been proposed up to now but any of them are not satisfactory from view points of performance, reliability and economy.
Generally, in the conventional anti-skid method as proposed, the acceleration of the wheel, as well as the deceleration, i.e. the negative acceleration of the wheel, is detected, and the braking torque is controlled by judging the possibility of the lock of wheel from the magnitudes of the acceleration and deceleration.
However, it is difficult to control under various conditions the braking torque in such a manner as to make the slip rate fall within an appropriate range, if the control relies solely upon the magnitudes of acceleration and deceleration of the wheel. It is therefore desirable to use the slip rate in a suitable manner as one of the controlling factors.
The term "slip rate" is used here to mean a rate which is given by the following equation: EQU .lambda.=1-Vw/V
where, .lambda., Vw and V represent, respectively, the slip rate, peripheral speed of the wheel and the speed of vehicle.
As will be understood from the above equation, there is no direct relation between the slip rate .lambda. and the acceleration Vw of the wheel which is obtained by differentiating the peripheral speed Vw of the wheel. It is therefore necessary to detect the speed of the vehicle body, for determining the slip rate of the wheel during braking.
Various methods have been proposed for detecting the speed of the vehicle body, i.e. the vehicle speed: i.e. (1) to use a Doppler radar mounted on the vehicle, (2) to detect the vehicle speed from the peripheral speed of a specific non-braked wheel and (3) to calculate the vehicle speed through integrating the acceleration and deceleration. These conventional measures, however, inconveniently require complicated constructions of devices and have poor precision and reliability. Thus, it is not easy to obtain a method which affords the detection of vehicle speed in a satisfactory manner, from technical and economical points of view.